IDE support came from the community as an intellij addon, which jetbrains then officially sponsored. But they didn't decide to throw their own resources at making such a thing from scratch.
Jetbrains has sponsored an alternative to the RLS, Rust Language Server.
The RLS is used in VSCode, I think Atom, I’ve seen a port for Emacs, and I think Sublime.
The community effort is behind the RLS, the Jetbrains plugin for IntelliJ doesn’t use any of the Rust compiler, as far as I’m aware, but it does have a lot of fans, so they’re doing something right.
As a Java fan of IntelliJ, I still prefer the RLS+VSCode, but it’s great seeing all the IDEs being developed for Rust!
Support from a big corp is one way to get things like IDE support ironed out, as they can throw extra resources at it.